1. Field of the Invention
The present invention relates generally to methods of measuring the concentration of an analyte in a fluid sample. More particularly, this invention provides method and apparatus for detecting the presence of a test element, such as a dry chemistry strip, in a meter device.
2. Background of the Invention
Monitoring analytes such as glucose, cholesterol, intoxicants, and other constituents is frequently desirable in fluids, such as blood, plasma, blood serum, saliva, urine, and other biological fluids. In healthcare applications, such monitoring affords the opportunity to make rapid diagnoses of a patient""s condition and to take prophylactic or therapeutic measures necessary for maintaining proper health.
One such healthcare application that has benefited tremendously by analyte monitoring in recent years is the treatment of diabetes. Diabetics suffer from an impaired ability to regulate glucose levels in their blood. As a result, diabetics can have abnormally high blood sugar levels known as hyperglycemia. Chronic hyperglycemia may lead to long-term complications such as cardiovascular disease and degeneration of the kidneys, retinas, blood vessels and the nervous system. To minimize the risk of such long term complications, diabetics must strictly monitor and manage their blood glucose levels.
Diabetics that have glucose levels that fluctuate several times throughout the day require very close blood glucose level monitoring. Close monitoring of blood glucose levels is most easily obtained when a diabetic is able to monitor their glucose levels themselves. Many devices currently available allow diabetics to measure their own blood sugar levels.
Reflectance-based monitors comprise one category of personal, or home-use, glucose level monitoring devices. These monitors utilize an optical block which accepts test elements for photometric analysis.
The test elements are usually in the form of test strips, which contain analytical chemistry. Conventionally, these test strips are in the form of a disposable diagnostic test strip containing analytical chemistry upon which a fluid sample is deposited. Once the user applies the fluid sample to the test strip, and the sample has sufficiently penetrated the test strip, a chemical reaction occurs in the presence of a target analyte, e.g., glucose, to cause a change in the optical properties of the test strip. An optical photometric device then determines the analyte level of the sample by measuring an optical property, such as the intensity of reflected light at a certain wavelength from the test strip. For in vitro analysis in healthcare applications, the fluid sample is usually fresh whole blood.
Diagnostic test strips for testing analytes such as glucose levels of blood samples are well known in the art and comprise various structures and materials. Test strips typically include single or multi-layered porous membrane arrangements which receive a blood sample and undergo a change in an optical property, such as a color change, in response to the interaction of blood glucose with agents/reactants in the membrane. Examples of such multi-layer strips are described in U.S. Pat. Nos. 5,296,192 to Carroll and 6,010,999 to Carroll et al., the contents of both of which are incorporated herein by reference.
Prior to reaching the reactants, a whole blood sample can be filtered to eliminate potential optical interference by removing erythrocytes, or red blood cells. Some test strips operate to allow the applied blood sample to migrate to a reaction site in the membrane where the sample reacts with the agents/reactants, which is located in downstream capillary relation to the sample application site. The results of the reaction are often visible as a color change at the reaction site. However, the change may occur in invisible regions of the electromagnetic spectrum, such as infrared and ultraviolet. For the purposes of this application, the term xe2x80x9ccolor changexe2x80x9d will be understood to include variations in optical properties throughout the visible and invisible regions of the electromagnetic spectrum. As noted above, a color change can be correlated to the amount of glucose in the sample. Home-use glucose measuring devices that use a reflectance meter to measure the color change of the test strip correlate glucose levels to the change in the amount of light reflected from the reaction site of the test strip. As is well known in the art, strips can be formulated to produce a color change within a certain spectral region, and the meter designed to photometrically measure reflected, absorbed or transmitted light at a wavelength sensitive to the color change of the strip. While the present invention will be described with reference to reflectance based photometry, it would be known to one having ordinary skill in the art to apply the features of the invention to absorbance or transmittance based systems.
An important aspect to the accurate measurement of glucose levels in a fluid using a test strip are the methods used to calculate the glucose concentration values from the reflectance values obtained. Because different samples physically vary and will contain different levels of analyte, reaction rates and durations will vary. Prior art devices have focused on fixing an initiation point, the time from which the monitoring device begins to measure the chemical reaction of the blood sample with the test strip. This initiation point often was carefully tied to the initial contact of analyte and reagent, either manually or automatically and then the reaction was timed for a fixed period of time from this initiation point. The end point is the time at which the monitoring device takes a final reflectance reading to calculate the reported glucose level of the sample from calibration data stored in the meter""s memory. Because a fixed time (or times) is used in the prior art, calibration is simplified, but this approach requires waiting for a fixed period. The fixed time period is usually longer than required for the reaction to complete, resulting in user inconvenience.
Some home-use glucose monitoring devices have an initiation point corresponding to manual, or user determined events. For example, some monitoring devices trigger the initiation point for measuring glucose levels upon the pressing of a button, the insertion of a test strip into the monitoring device, or upon closing an element, such as a cover or door, of the monitoring device over the test strip. These user-defined initiation points decrease the accuracy and consistency of the monitoring device because they rely on the inconsistent timing of an action by the user (i.e. insertion or covering of the test strip in the monitoring device). These inaccuracies in determining the initiation point are commonly carried through to the end point measurement time. This results from the fact that many common monitoring devices use a fixed time period from the initiation point to determine when to initiate the end point measurement. This fixed period timing is especially problematic when using multilayer test strips because of the nonuniform absorption periods inherent with such test strips, owing to physical differences between various samples (e.g. hematocrit, sample viscosity, as well as general operating conditions such as humidity, temperature, etc.).
Accordingly, conventional methods for determining initiation and end points for measuring glucose levels from test strips may yield inaccurate results because the methods depend on events or time periods unrelated to the reaction kinetics of the blood sample and the test strip. Further, because reactions each occur at different rates, reliance on a fixed time period between an initiation point and the end point may prolong the measuring time beyond the time necessary to accurately measure the glucose level of the sample, resulting in user inconvenience. While a more accurate measuring time may only yield a few seconds improvement over a fixed measuring time, such an improvement is substantial to the person who uses a device multiple times daily. Further, a more accurate, uncomplicated and quick method for testing blood samples for glucose levels will encourage patients to monitor their blood sugar levels more regularly, thereby promoting compliance with their prescribed regimens for diabetes management. Such an improved method is described in commonly assigned copending U.S. patent application Ser. No. 09/794,045, filed concurrently herewith, the contents of which are incorporated herein by reference.
An endpoint seeking algorithm such as that described in the above patent application incorporated herein by reference begins when it is determined that an analytical element, e.g. a chemistry strip, has been inserted. A separate strip sensor signal is generated by a photo-reflective device or xe2x80x9cstrip sensorxe2x80x9d when a sample that is above a certain reflectance, such as the white tip of a PRESTIGE strip, is placed in close proximity to the sensor.
In ordinary use, photo-reflective strip sensors of the type used herein simply generate a current that is roughly proportional to the reflectance of the sample which is in it""s field of view. This current is converted to a suitably scaled voltage using an appropriate load resistor, and if this voltage is above a specified level, the decision is made that a strip is in place.
While this approach has been used successfully on prior art meters, there are some drawbacks.
First, there are part to part variations in the sensor itself. The sensitivity of economical photo-reflective sensors is not tightly controlled. The manufacturer of the part can select parts to reduce the spread in sensitivity, but at an increased cost.
Second, there are meter to meter variations due to variations in the mounting of the part during manufacturing, and optical variations due to surrounding plastic parts that can reflect some light toward the sensor. There is a limit to the uniformity of the manufactured optical assembly when all the components involved are considered.
Third, the threshold voltages of microprocessor inputs are frequently poorly specified. Even if the sensitivity variations cited are minimized, the threshold level and the part to part variability of ordinary microprocessor inputs are not often known with precision. Accurate thresholding circuitry can be added, but at additional cost.
Fourth, other component tolerances add variability to the sensitivity of the measurement. For example, tolerances on the current setting resistor for the device""s LED, tolerance on the device""s LED forward voltage, and tolerance on the device""s output load resistor all add further variability to the system""s sensitivity. In addition, the current setting voltage supplied by a microprocessor output can contribute additional variability.
Fifth, Standard Strips generate some reflectance signal, but must not be detected as a chemistry strip. Standard Strip tests are discussed further in commonly assigned copending U.S. patent application Ser. No. 09/794,044, filed concurrently herewith, the contents of which are incorporated herein by reference. During a Standard Strip test, a small portion of the standard strip will normally be within the field of view of the strip sensor and will generate a small amount of current due the light reflected from it. This condition, while small, will decrease the margin of error slightly for a fixed threshold case.
Sixth, ambient light leakage into the sensor that cannot be distinguished from light reflected from an inserted strip. Sunlight and light from interior lighting can reach the strip sensor and cause the sensor to produce a current that adds to the current produced by light reflected from a standard strip, a partially inserted test strip, or in combination with other internal reflectances. This can cause a false determination that a strip is in place. Tightening the tolerances in the plastic parts and altering the color and transmittance of the plastic parts can manage light leaks, but this adds a burden to the mechanical design and increases cost. Increasing the light that is used to make the reflectance measurement has reduced this sensitivity to bright ambient light somewhat in prior art meters, but the additional current required reduces battery life.
These drawbacks reduce the confidence that a simple fixed voltage level will always correctly identify that a strip is in place. Accordingly, it is an object of the present invention to provide an improved method for accurately detecting when a test strip has been inserted into the meter.
In accordance with the invention, a method is described for calibrating a strip sensor in an analytical meter device, the strip sensor having a photodetector and a light source, the method comprising the steps of: a.) measuring the voltage output difference of said photodetector between when said light source is on and off using a standard having relatively high reflectance; b.) measuring the voltage output difference of said photodetector between when said light source is on and off using a standard having relatively low reflectance; c.) calculating a voltage threshold VTH produced by the strip sensor to be used by said analytical meter device to indicate when a test element has been inserted therein according to the formula:
VTH=KTH[(VHighxe2x88x92VHighxe2x80x94Off)xe2x88x92(VLowxe2x88x92VLowxe2x80x94Off)]+(VLowxe2x88x92VLowxe2x80x94Off) 
where KTH is a constant; d.) evaluating the value of VTH calculated in step (c) for acceptability according to predetermined criteria; and e.) permanently storing VTH in said analytical meter device if evaluated as acceptable in step (d).
The present invention advantageously allows the use of lower cost strip sensor devices, without compromising performance. According to one aspect of the invention, the sensitivity of each device is measured during factory calibration and a threshold calculated for each individual device. Thus, a much broader spread of sensitivities in strip sensors can be accommodated without increasing the risk of the meter making erroneous strip insertion decisions.
The present invention also provides improved confidence that erroneous strip detection will not occur. According to another aspect of the invention, the complete strip sensor system has its sensitivity measured during calibration. Thus, there is less risk of making erroneous strip insertion decisions due to normal variations in manufacturing tolerances of other components in the system and due to variation in the assembly of the system.
The present invention also provides improved performance in varying ambient light conditions. According to another aspect of the invention, the sensor""s signal due to ambient background light is measured prior to each normal reading and this signal is accounted for separately from the signal due to reflection from an inserted strip. Thus, the likelihood that excessive ambient light will cause an erroneous strip insertion decision is reduced. Because of the highly improved light immunity, greater flexibility is afforded to the mechanical and industrial design.
The present invention also provides additional checks to ensure that testing is not performed in excessive ambient lighting conditions. According to yet another aspect of the invention, these ambient checks add one more layer of assurance that a chemistry test will not be performed under lighting conditions that could cause errors. These checks allow the meter to adapt to changing ambient lighting conditions.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.